The placental membrane preparation contains all the components of a cholinergic system; acetylchoinesterase, acetylcholine and choline acetyltransferase. This model system also exhibits active amino acid uptake and seems to contain a muscarinic receptor. Although this cholinergic system is in the human placenta, there is no innervation to this organ. This placental vesicle model appears to be useful and reliable model to study human placental transport as well as factors and agents influencing this transport. We propose the placental cholinergic system has two possible functions 1) to regulate transport from maternal to fetal circulation of ions, energy metabolites and other endogenous substances for growth and 2) the release of placental hormones required for the maintenance of pregnancy. This cholinergic system may also be related to the placental barrier against the transport of foreign substances, drugs, and environmental toxicants. An "integrating concept" or more generalized concept of cholinergic function in excitable membranes has been proposed. It is from this theoretical model that one can postulate a cholinergic role in the basic homeostatic control of membrane potentials, similar to that seen with Na/K ATPase, being coupled with the metabolic pathways of the cell. The existence of a cholinergic receptor on the syncytiotrophoblast membrane further supports the hypothesis of a functional role for the cholinergic system in placental function. Effects of environmental toxicants on this cholinergic system may explain the embryo-toxicity of a variety of pesticides as well as heavy metals.